98 k1500 a/c heat vent control problem

I have a 1998 Chevy K1500 6.5L turbo diesel and my problem is that the a/c heat vent control isn't working properly. I have spent 2 full days trying to fix it myself but with no luck. The vents wont change from floor to any other position the knob twist but no effect and the a/c and recirc air buttons don't engage or light up. What I've tried is to replace the control unit but it made no difference and to check the mode door motor, works but only when the fan/blower is turned off so it will change the vents but as soon as you put the blower on the vents go back to floor, also I bypassed the control unit and hot wired the a/c to see if it still worked and with the blower going and the control unit bypassed the a/c blows ice cold. so my only guess is a relay. This is the hvac control unit with 3 twist knobs and a button for a/c and a button for recirculating air. I replaced the control unit when I first got the truck 2 years ago b/c the controls would stay on the windshield vents and wouldn't change to any of the other vents and the a/c button wouldn't light up or engage the a/c. Replacing the control unit fix it all. But now it is something different any help or input you can give is well appreciated.